1. Field of the Invention
The present invention relates to a recording material and particularly, to a recording material in which a novel color developer is employed.
2. Description of the Prior Art
Recording materials utilizing substantially colorless color formers which can produce color upon contact with acidic substances (a color developer) are well known. Specific examples of such recording materials include; for instance, pressure sensitive copying paper, heat sensitive recording paper, electro thermo-recording paper and so on, and they are described in detail in U.S. Pat. Nos. 2,712,507; 2,730,456; 2,730,457; 3,418,250; 3,432,327; 3,981,821; 3,993,831; 3,996,156; 3,996,405 and 4,000,087; etc. In these recording materials, clay minerals such as acid clay, bentonite, kaolin or the like; isopropenylphenol dimer; phenol-formaldehyde novolak resins; metal-processed novolak resins; di-tert-butylsalicylic acid; etc. are used as color developers. As a color former, compounds which are usually substantially colorless and have in their skeleton a lactone, a lactam, a sultone, a spiropyran, an ester, an amido or like structures, which undergoes ring opening or bond fission upon contact with a color developer, have been employed. Specific examples of such color formers include Crystal Violet lactone, benzyl leuco Methylene Blue, Malachite Green lactone, Rhodamine B lactam, 1,3,3-trimethyl-6'-ethyl-8'-butoxyindolinobenzospiropyran and the like. These color formers may be used in combination depending upon the end use.
When the above described color developers are employed in a recording material, the recording material lacks water resistivity, has low keeping stability, has a high melting point and consequently low workability, color contamination appears when it is dipped in water, special treatment is required to enable the color developer to fully exhibit its color developing ability and others. For instance, when a salicylic acid derivative such as 3,5-di-.alpha.-methylbenzylsalicylic acid or the metal salt thereof is employed as a color developer, it is known that it does not exhibit its full color developing ability unless it is subjected to a sand grinding mill treatment. Overcoming these disadvantages is an important subject of this art.